What went wrong ? It turns out that the Adwaita css file is pretty minimal:

@import url("resource:///org/gnome/adwaita/gtk-main.css");

All the secret sauce is wrapped up in a resource bundle in the same directory. GTK+ automatically looks for such a bundle in the same directory as the css file when loading themes. With this knowledge, making my local copy of Adwaita work is a simple as linking the resources into the right place:

Editing CSS? You’re not serious with this, are you? Such changes should be done by either using a ‘Edit’ menu or by simply dragging the separator and making the sidebar smaller directly in the application.
Oh, wait, being able to do this without editing CSS files might be too confusing for users, I see…